1. Field of the Invention
The present invention generally relates to digital communication networks for two-way communication of data signals of different media such as voice signals, data signals, image signals, etc., based on a multiplexing method such as frequency-division multiplexing, where a single communication line extending from a center device is connected to one or more communication terminal devices.
The present invention particularly relates to an automatic adjustment of carrier levels, transmission of acknowledge signals, and an automatic adjustment of signal-transmission timings between the center device and the communication terminal devices connected with each other via the communication line on the digital communication network.
In the field of communication technology, an intensive research effort has been directed to two-way communication of data of various types on a network system which connects a center device and a plurality of communication terminal devices via a shared communication line. An example of such a network includes a CATV transmission system which has been used in the field. In the network, levels of carriers which the center device or each of the communication terminal devices transmits to the communication line should be set to a predetermined level by taking into consideration the signal loss along the communication line. The present invention relates to an automatic adjustment of the carrier level of each communication terminal device.
2. Description of the Prior Art
Typically, a carrier level of each communication terminal device is fixedly set to a predetermined carrier level which can compensate for the loss along the communication line. The determination of the carrier level is made after an examination of various aspects including the distance between the communication terminal device and the center device (i.e., after laying out a loss plan). It is generally preferable that a level difference between signals received at the center device is within a range of 6 dB. As for transmission from the center device, the carrier level is fixedly set to a predetermined level on the basis of the distance between the center device and the farthest communication terminal device.
In contrast to the fixed carrier level, an automatic adjustment of a carrier level for the communication terminal devices has been proposed. For example, the Japanese Patent Laid-open Application No. 63-119330 discloses a level adjustment which is carried out only at the time of receiving a packet sent from a communication terminal device (i.e., only at the time of data transmission). This level adjustment has been proposed to counter a problem in CSMA/CD (carrier sense multiple access with collision detection) used in the broadband LAN, where an existence of a communication terminal device having an inappropriate carrier level makes it impossible to detect a packet collision so as to incapacitate communication with other communication terminal devices. In this level adjustment, a communication terminal device needing the level adjustment contacts the center device, and, then, the center device sends back a level detected at the center device. Based on this received level, the communication terminal device adjusts the transmission level.
Depending on conditions of an environment of the communication terminal devices and the communication line, however, the loss of the carrier level may deviate from the predetermined loss plan, or may vary over a passage of time in a short run as well as in a long run. Also, it is possible that the carrier level itself changes. In these cases, the center device may detect these deviation and variations as an error, or may even be unable to detect the carrier.
The level adjustment method disclosed in the above reference, however, measures a level of the received signal only after a communication terminal device requests the level adjustment, and, then, sends the detected level back to the communication terminal device. Thus, factors varying over a passage of time cannot be tracked to carry out an appropriate level adjustment.
Also, since the communication for the level adjustment is conducted via an ordinary communication channel, it may affect communications of other communication terminal devices.
Furthermore, when a frequency band is divided into a plurality of high-frequency (RF) channels having equal bandwidth so as to carry signals such as voice, data, images via each channel, a determined carrier level of some channel may have an adverse effect on other channels, depending on which channel is used.
The level adjustment method disclosed in the above reference does not take into account these problems.
Accordingly, there is a need for a method and a device for an automatic carrier-level adjustment which can adjust the carrier level for each channel by following actual level fluctuation without affecting ordinary communication.